The invention relates to an operational transmission comprising highly loaded gearwheels, whose housing forms a sump which contains a lubricant and coolant and into which at least one gearwheel dips. Such transmissions occur in a wide variety of applications, in particular in motor vehicles, as a change speed gear mechanism, distribution transmission of all wheel drive vehicles and as an axle-mounted or differential transmission.
In the case of highly loaded gearwheels, even when they are lubricated in the best possible way dissipated heat is generated which has to be carried away. This is done either by means of a coolant and lubricant circuit with a pump and an external radiator or by dissipating heat to the surroundings via the housing of the transmission. For this purpose, said housing is to have the largest possible surface. In motor vehicles, said housing is usually provided with external ribs in order to increase the surface for the transfer of heat to the passing slipstream. If the power of the engine is to be increased without more installation space being made available for the transmission, a limit is quickly reached.
At present, lubricating oils are used exclusively in transmissions, but completely synthetic oils are usually used in highly loaded ones. However, all the oils (as used herein the term “oil” comprises petroleum fractions, vegetable oils, or synthetic liquids such as hydrogentated polyolefins, esters, silicones and fluorocarbons) have the following disadvantages in common:
a relatively small thermal capacity, conductivity of heat and low coefficient of the transfer of heat (for the sake of comparison, approximately half that of water),
the viscosity is highly temperature-dependent, and losses are increased as a result of this in the cold state and consumption of fuel is consequently relatively high,
not environmentally friendly (disposal!) and not resistant to ageing over the long term.
For this reason, consideration has already been given to alternatives for a long time and for this purpose trials have been carried out without alternatives being proven in practice. In this context water was considered appropriate because of its excellent thermal properties. Its viscosity is largely constant, which is, on the one hand, an advantage, but, on the other hand, is not sufficient for the formation of a stable lubricating film between the tooth edges. An attempt has been made to remedy this through suitable coating of the gearwheels. Whatever coating has been attempted in the past, it was either not able to cope with the tooth edge pressure or the sliding speed or it disappeared completely after only a short operating period. This has also been found in the case of carbon coatings.
The search for alternatives is also reflected in the patent literature. For example, U.S. Pat. No. 4,714,414 discloses lubricating the gearwheels of the transmission stage of a compressor with sprayed water for oil-free compressed air. The fact that the gearwheels can be composed of plastic here indicates a low power to be transmitted. These are therefore not highly loaded gearwheels.
In the DE patent 196 05 162 C1 a synthetic working fluid is described which is composed essentially only of an alkylene glycol to which corrosion and oxidation inhibitors are added. Although the thermal properties of an alkylene glycol are better than those of oil, they do not approach those of water. In particular, the viscosity of water is higher and it is also dependent on temperature.
The object of the invention is to provide an operational transmission which avoids the disadvantages of lubricating oil and makes use of the advantages of water to the highest possible degree, without the disadvantages of water.